The proposed research is directed toward understanding 1) the factors which modulate the differentiated state of chondrocytes, and 2) the hormonal regulation or cartilage matrix synthesis. We have observed that dedifferentiated chondrocytes will return to the differentiated state if changed to spherical shape in agarose suspension culture. Cytoskeletal involvement in phenotypic switching is indicated because dedifferentiated monolayer chondrocytes will reexpress the differentiated collagen phenotype after treatment with the microfilament-disruptive drug, cytochalasin B. We will document and expand this information on a microfilament-dependent pathway for gene regulation in chondrocytes. We have hypothesized that the increased rate of proteoglycan (PG) and collagen synthesis (8-fold) that occurs during in vitro culture of articular cartilage slices is due to an increase in the number of somatomedin receptors per cell. Following an initial lack of serum dose response, original chondrocytes and their progeny in agarose, monolayer, and slice cultures become dose responsive as measured by PG and DNA synthesis. Radiolabeled growth factors will be used to probe for parallel increases in receptor number/cell. If we can understand the mechanism of this increase in cellular responsiveness to serum we may be able to increase matrix synthesis in osteoarthritis and greatly decrease the cartilage destruction that follows proteoglycan depletion. These studies will be performed with rabbit humeral articular cartilage and chondrocytes. Collagen phenotypes will be routinely determined by SDS electrophoresis and 2-D CNBr peptide maps of isolated collagens. Understanding the mechanism that stabilize the chondrocyte phenotype and enhance matrix synthesis may facilitate the design of therapeutic strategies that stimulate resistance to the initiation and progression of osteoarthritis.